Electrical insulation material



106. cowosmows,

Examiner UNITED STATES PATENT OFFICE ELECTRICAL INSULATION MATERIAL William R. Heineman, Meriden, Oonn., assignor to General Electric Company, a corporation of New York No Drawings.

6 Claims.

The present invention relates to electrical insulation material. It is concerned more particularly with the preparation of an arcand moisture-resistant asbestos board which may be drilled and machined fithout damage to the tool and with practically no chipping or cracking of the board.

Asbestos board consists largely of asbestos bonded with a suitable inorganic binder such, for example, as ggrtlapg cement. Such a board, in the form of sheets ranging from about $4; to 2 inches in thickness, has wide use in the electrical industry, where it is employed chiefly as a switchboard panel material. Numerous holes must be drilled in these panels for wiring the switchboard. It is therefore highly desirable that the asbestos board wt ready drilling without excessive wear of the armana with Tittle or no chipping around the holes. Further, the board must withstand moisture and the action of an electrical arc.

Heretofore in making an arcand moistureresisting asbestos board, it has been common practice to impregnate the hardened board by immersion in a suitableplly Ol waxy impregnating agent, such as a hot mixture of firnauba and mama waxes, for from 2 to 4 days or more. Such a method is described more fully in, for

a. instance, Groten Patent No. 2,049,370, which is assigned to the same assignee as the present invention. Asbestos boards impregnated in accordance with the Groten method have been satisfactory from the standpoint of arc and moisture resistance, but have not been wholly satisfactory with respect to drilling and machinability. Further, the method had the objection of requiring considerable time for impregnating the board with the wax mixture.

I have discovered that a dense, arcand moisture-resistant high dielectric strength asbestos board, which is adapted to be readily and elliclently drilled, can be obtained by forming and hardening the board in the presence of, that is while intimately associated with, a high meltin waxy or wax-like substance such, for example, as montifi wax, carnauBa wax, ozokerite, or.

similar ma er a amltingm at leastJbQjJ, and preferably melting above 160 wdre fiarticularly it may be stated that I incorporate such a waxy material or mixture of waxes into a mass comprising h draulic cement, asbestos and water. Thereafter a large part of mess waEr is' removed coincidentally with the shaping of the plastic mass, after which the Application July 23, 1938, erial No. 221,025

hardened. The cured g r hardened product is sanded smooth. It may be used directly, or after being suitably painted or varnished, but in no case does it require immersion for a prolonged 5 period in an impregnating agent as heretofore has been necessary.

In order that those skilled in the art better may understand how to carry the present invention into eifect, the following more detailed descriptlon of the manufacture of a switchboard panel is given:

Suitable proportions of an inor anic fibrous material such as asbestos, h drafihc cement such as Portland nt, a wax 0; high melting point,

pre'ferably mpn tan wax, and water are thoroughly mixed in a suitable mixing apparatus until a homogeneous mass has been obtained. Advantageously there is also incorporated into the mixture colloidal clay such as bentonite or colloidal kaolin. The addition of the colloidal material provides a denser asbestos board. It also has been found to be advantageous to add a small amount of ra hite to the mixture. The graphite acts as a lu cant and facilitates molding of the material. Further, it improves the arc resistance and other electrical properties of the board. A convenient mixing technigueis to mix thoroughly the cement, clay, warts-tamer, for instance for about 15 minutes. The asbestos is added and the whole mixed for an additional 15 minutes. Sufllcient water is used so that a slur- Wresults.

ore a cularly it may be stated that best results have been obtained with the use of the following components within the approximate limits stated:

Parts by weight Water sufficient to form a slurry.

Preferably colloidal clay is used within the limits of 3 to 7 parts and graphite, within the limits of 0.5 to 1 part.

In molding the mixed ingredients a screen of approximately 20 mesh size is placed in the bottom of the mold, the material is flowed thereon, and another screen of approximately the same size is placed on top of the wet mass. Upon the application of pressure the water is freed from shaped mass containing the dispersed wax is the mass, flowing out from a sin a e b p'filng in the bottom of the mold, and also from the top of terial, it is to be understood that other equivathe mold between the punch and the jacket. By lent materials may be used. For example, in using screens in the manner described, the larger lieu of all or a part of such high melting waxes, part of the excess water is readily removed from wor resins or s thetic resins, cathe mass. If water he allowed to remain in the 5 pable of improving the drfilir ig and machining panel, the finished board has poor impact and properties of an asbestos-cement composition in nexural strength characteristics. On the other cured state, may be employed. hand, it is very desirable to have an excess of What I claim as new and desire to secure by water in the mix as it is introduced into the tters Patent of the United States is: mold. By so doing, the asbestos fibres are not 1. An electrical insulation material having disknotted or broken during the molding operation. persed therethrough a waxy substance having 9. Instead, they form an interwoven mass which melting point of at least 150 F., said insulation has been found to be essential in obtaining optimaterial consisting of the pressed and hardened mum mechanical strength. product of a mass comprising asbestos, hydraulic The molded p rt are air-dried for a u bl l5 cement, water and the said s tance disperiod, for instance for a u to 5 hours. e e y: Thereafter they are placed in an oven in which rend steam is constantly introduced. The panels are i cured in this oven for a suitable period, for exmpl f r a t 24 hours. After the e m we 2. moisture-resisting switch-board panel they are immersed in warm water (1m capable of being drilled without chipping and F.) for mififile interval, for insta ce having dispersed therethrough a wax having a for about 24 hours. Following this treatment melting point of at least 150 F., said panel comthe panels are baked in a suitable oven, such for prising the product obtained by dehydrating and example as an electrical oven, at a gradually ris- 25 hardening a slurry comprising water carrying ing temperature up to approximately 500 F over particles of asbestos, Portland cement, a colloidal a period of, for instance, about 12 how. KY1 the clay and a wax having a melting point of at least water is removed from the panels by 150 F. dispersed throughout the said slurry. "baking" reaEm e components are 3. A moistureand arc-resistant electrical inbonded together to form a hard, rigid mass. sulation material especially adapted to be drilled The baked panels aremdgito a smooth finish, without chipping around the resulting hole, said after which they usually are sprayed or othermaterial having montan wax dispersed therewise coated with a suitable lacquer or varnish. through and being the pressed and hardened By incorporating a high melting w'ax such as product of montan wax or the like into an asbestos-cement P rts by weight, composition in the manner described, the wax Portland cement 12to 18 doe not melt until the cement has more com- Asbestos 7 to 11 pletely reacted (set up) in the steam-curing oven. Montan wax t 2 During curing, the dispersed wax melts and coats Colloidal clay Up to '7 th individual articles oi mass' withjfwax 40 ,Graphite Up to 1 Thoroug i coating of the particles with Water suflicient to form a slurry.

th''wax is further assured by the oven-baking treatment.- The ultimate result oi thiswax coat- The method 0! improving the illing and ms of the particles is to improve the water machining characteristics of an electrical insulasstance of the finished board In addition the 5 tion material consisting of a shaped, hardened presence of the wax surprisingly improves the mass comprising asbestos bonded with hydraulic arming and machining properties of the finished cement which consists in bonding the said matepanel. For example, when effort was made to rials while wax having 8 melting P nt of at drm a wax4ree board with an ordinary yrmch least 150 F. is dispersed throughout the plastic drill, the drill was rounded over at the point and mass comprising asbestos and Y 311110 t. was completely worn out without drilling through A method in claim 4 wherein the ax is the board. In contrast therewith, an asbestos montan board containing approximately 4 per cent. by The method of preparing a high dielectric weight of the whole, of montan wax could be strength moisture'resisting insulating m t rial readily drilled without burning or otherwise capable of being drilled without chipping, said luring the drill and with practically no chipping method rising forming a i t at around the drilled holes. The finished panels, in P rt b h general, have a dielectric strength of the order Portland cement a s gi t or 45 to 55 volts per mil. Further, the material Asbestos 7 shows no surface carbonization under the in- Mont Q5; i fluence of an electrical arc, so that during service Colloidal cla'y 6 use there is no material depreciation of its in- Graphite gzgg sulation value. Representative samples of panels (2" x 2" x 1") absorbed only about 0.5 to 1.1 water sumciem w mm a Slum" per cent water when immersed in water for 24 e5 ving the larger p i the wa r from the hours, and only about 0.9 to 1.5 per cent when said mixture coincidentally with the shap f immersed for 48 hours. The average Brinell the mass, air-drying the shaped mass, steamhardness, under a 500 kilogram load applied for curing the air-dried mass, soaking the steam- 10 seconds, generally ranges from about 20 to 27. c red mas in warm water, and baking the water- The average transverse strength usually is about Soaked m ss t a gradually rising temperature 4000 to 5000 pounds, or more, per square inch. up to approx mately 500 F. until substantially whil I hav d rib d my i v tio with arall the unoombined water has been removed and ticular reference to the formation and hardenthe components have been bonded together into ing of an asbestos-cement composition in the a h d, rigid m presence of a high melting waxy or wax-like ma- WILLIAM R. HEINEMAN. 

